Résumé
Ferroelectric nano-objects display a rich variety of shapes, among them nanocubes (bulk or hollow),nanospheres, nanotori etc with fascinating morphogenesis mechanisms1. Structural and physicalconsequences of ferroelectric downsizing to form nanopowders or nanoceramics are very important bothfrom applicative and fundamental point of view (see ref. 2 for a good review). Indeed many interestingeffects have already been reported such as diminishing of permittivity and increase of diffusivity2,3,cancelation of dielectric relaxation in relaxors3,4, lowering of ground states, increase and rotation ofpolarization in relaxors and morphotropic compounds3,5 etc. In order to explain these experimental results,one must take into account not only size and microstrains effects but also existence of core-shellstructure2,5-8. Modeling also shows that there is optimal nanostructurations to maximize energy storage6 andelectrocaloric effect8. Calculations also show the possibility to form new phases in which vortices ofpolarization compete with homogeneous polarization8 or in which homogeneous hypertoroidization ofnanotori coexists with axial homogeneous toroidization10. After a survey of these effects, we will give newexperimental results observed in SrTiO3 showing at small size decoupling of oxygen octahedra tiltings anddistortion, strong shift of ferroelastic critical temperature11,12 and lowering of the tetragonal structuretowards orthorhombic ground state. Possible connection with a core-shell structure will also be discussed.